Automatic tone control circuit



March 9, 1937. c. TRAVIS AUTOMATIC TONE CONTROL CIRCUIT Filed Jan. 28, 1935 Fly 2 INVENTOR CHARLES TRAVIS ATTORNEY Patented Mar. 9, 1937 UNITED STATES PATENT, OFFICE,

AUTOMATIC TONE CONTROL CIRCUIT Charles Travis, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application January 28, 1935, Serial No. 3,702

13 Claims. (Cl. 250-20) My present application relates to automatic its high frequency components, which seem to be tone control circuits, and more particularly to entirely responsible for the disagreeable effect such circuits employed for noise suppression in produced upon the ear. The fact that the lower automatic gain regulated receivers. frequency components of the noise come through Automatic tone control circuits (ATC) have to some extent is an advantage rather than other- 5 been developed to perform the function of atwise, for it reminds the listener that the set is tenuating high audio frequencies to-agreat degree operating at all times in a sensitive condition. with decreasing signal strength. This function If the set cuts off entirely between stations the is extremely useful in the case of very weak siglistener is likely to fear v at times that he is missing 19 nals (Well down into the noise level), and inter- Weak stations through lack of adequate sensil0 station noise in the absence of carrier. Here tivity. the action of the circuit renders the interference One of the main objects of the present invenmuch less disagreeable to the listener by retion is to increase the flexibility of the automatic moving the high frequency components, and the tone control action of the AT0 tube whose input circuit becomes in effect a muter device, or noise admittance is utilized for the control action, 15

suppr ss r. u h a syst a been d s s d y and to accomplish this flexibility increase by a Jacob Yol es n application Serial No. 638,514, relatively simple change in the tube circuit elefiled October 19, 1932 patented October 15, 1935 ments. as U. S. P. 2,017,270. Another important object of the invention is 20 In principle the circuit shown in the said apto provide a muting tube for a receiver employ- 20 plication makes use of an effect in a triode, by ing AVC, wherein the muter tube includes virtue of which the input admittance of the tube capacity and resistance in series between its in fl p p the Values 0f the plate to gr d put grid and plate, whereby the proper value of capacity, the external plate load and the mutual plate resistance may be chosen to suit the range 2-3 conductance. A tube is connected with its gridf control desired, as well as the range of AVG a d p across the a d frequency lin bias strengths over which this control is to act, a d a its grid to plate p y gmented by while maintaining the initial value of the total an external fixed condenser; an appropriate load resistance at a sufiiciently high value. being placed in the plate circuit. The mutual Other objects are to improve "generally the 0 conductance is then vari dautomatica l by auto- 1 simplicity, e'fiiciency and flexibility of automatic 80 matic gain control bias applied to the grid through a high resistance filterl With'minimu'iri control bias, which is the condition for veryweak signals, "the' iripu e "maxim ant c frequenci "low one tanej seereases :40 ince, a'ndles que'nc es t'akesblac attenuate'higl'ffreduenci 'wak sig'nal'sthan 'scrited'g biit th U "that attenuatiorn ales-'-'place "only for -very" signai's and for noisein the"'absence" of carrier For tone control actiori' 'itis preferred toiextend therange' of signal s'trength iniwhicli variableiat- 5 tenuaticiitakes place}? when actin'g yas noise vantages. oiibe suppress'o-rithe*circuitidoesnotieliminate the-noise the? presentiinvention';zi entirelygibut rathertirenders the fnoise; most; tolere. simplifiedscircuit embodying -5 'able to theslistenerzby the completetextinction eof '32oiissthe equivalentt circuit 5 5 arrangement. Considering both these figures, it will be observed that the tube T and its associated circuits are connected across the input electrodes of the tube T1 (an audio amplifier fed with audio energy from source S). The input admittance of tube T is in series with condenser C, and between the grid and plate of tube T is connected the augmenting condenser C and re sistor R2. The plate load of tube T is a resistor R1; the grid bias of tube T is to be varied, as by AVC, to control the shape of the frequency response characteristic of the system including tube T1.

Without entering into any extended analysis of the relations shown in Fig. 2, it is pointed out that the symbol A is a quantity representing the voltage gain of tube T functioning as an amplifier with the coupling impedance (C-Rz) removed. This quantity may be complex (if the effective plate load is reactive), but in the case of a resistive plate load, A and (1+A), are real numerics. Hence, it will be observed that the magnitudes of the variable condenser and resistor, in series with condenser 0' across the input of tube T1, are functions of the gain of tube T.

Great flexibility of ATC action may be obtained by varying the values of a few physically simple circuit elements, as will be seen from the following considerations. for a simple fixed condenser between the plate and grid of the ATC tube, the variable impedance shunted across the audio source S is a resistance-condenser combination with an invariable time constant. With this arrangement the high audio frequencies only will be attenuated when the AVG bias is such that the tube is operating near its cut-off point, which occurs with weak signals; but with minimum bias on the tube, as in the absence of carrier, strong attenuation will occur for all frequencies but the lows. This is a highly desirable condition if interchannel noise suppression is the primary object. For a tone control as such, it may be preferred to limit the maximum value of the variable condenser, while permitting the variable resistance to decrease as far as it will. This may be accomplished immediately by properly choosing the size of the grid blocking condenser C of Fig. 1. Thus by making C small and C large, the tube becomes equivalent to a fixed condenser in series with a variable resistance.

The initial value R of the resistive portion of the combined variable impedance is not limited to what may be used as a plate load; it is augmented by adding resistance in series with the grid to plate condenser C. This is shown in Fig. 1, with its equivalent in Fig. 2. The advantage to be gained by doing this lies in the fact that it permits the proper value of plate resistor to be chosen to suit the range of control desired, and the range of AVG bias strengths over which this control is to act, while maintaining the initial value of the total resistance at a sufficiently high value.

Thus, with the usual audio impedance levels out of the manual volume control, an initial resistance value in series with the variable condenser of about one megohm is desirable. This as a plate load for a type 6D6 tube, which is the tube commonly used, with 250 volts plate supply, permits a screen voltage no higher than 10 volts, and although high values of A are reached (over 100) the grid bias at which the tube cuts ofi is only about 8 or 9 volts. These circuit conditions are probably nearly optimum, if noise suppression is It can be shown that supply.

the primary object, but if a wider range of grid bias is desired for tone control, it will be necessary to reduce the value of the plate resistor R1. The desired initial value of the resistance in series with C is maintained by placing the proper resistance in the position of R2.

A receiving circuit employing the arrangement of Fig. l is shown in Fig. 3. This figure shows the portion of a superheterodyne receiver following the I. F. amplifier and preceding the second A. F. amplifier. The network preceding the tube I (which may be of the well known multiple duty type may comprise one, or more, tunable signal amplifiers followed by a converter stage, (of the composite local oscillatorfirst detector type or of the independent tube type); the I. F. output of the first detector stage being amplified in the usual I. F. amplifier. One or more of the signal transmission tubes preceding tube l are usually adapted to have a variable gain, the AVG functioning to control the gain of these tubes. Such gain control circuits are well known at the present time, and any conventional system may be employed.

The tube I has its diode anode and cathode coupled across I. F. input circuit 2; the load resistors R8 and R5 being connected in the low potential side of the diode second detector circuit. A condenser C2 is arranged across Ra and R5 to by-pass the I. F. component of rectified current. Condenser C3 grounds the junction of resistors R8 and R5. The audio component of the rectified current is impressed on the grid of tube I through a path including condenser C4 and resistor Re. The fixed bias for the grid of tube 1 is provided by connecting the grid to the grounded side of voltage supply bleeder P through a path including resistors Re and R9. The cathode of tube i is connected to ground through resistor portion R10 of bleeder P.

The ATC tube is of the BBB type, and has its cathode connected to a point on the bleeder P about one volt higher than the potential of the detector cathode. The resistor portion R7 is connected between these points. This provides a minimum starting bias on the ATC tube of one volt plus the contact potential of the diode. It however is simpler, and almost as satisfactory, to connect the two cathodes directly together. The additional starting bias reduces the possibility of the reversal of ATC on very weak sig nals. The screen voltage is to be fixed so that maximum gain occurs at this minimum control grid bias. Too high a screen voltage (for a chosen value of R1 and plate supply voltage) is found to give maximum gain at some more nega tive bias value, falling off thereafter as the bias approaches zero. This gives the undesirable condition that the ATC action is less for very weak signals than for somewhat stronger ones. If R1 is made 1 meg, (R2 being omitted), it is found that 10 volts is proper for E02 with a 250 volt Smaller values of R1 permit higher screen voltages and thus wider ranges in control action, with some loss in maximum voltage gain and hence in maximum ATC action. This latter is not serious down to values of R1 of 100,000 ohms. For the smaller values of R1 it may be desirable to make use of the resistor R2, as has been described above.

Resistors R3 and R4 may be 1 to- 2 megohms, with C1 equal to .05 to .01 mi. as a filter condenser. It is to be noted that the audio signal from the manual volume control is only a portion of that detected. For usual volume control settings the A. C; on the ATC grid is thus only av small portion of the D. C. bias, which minimizes distortion. 7 a

The manual volume control resistor Rswill ordinarily be of the order of 0.5 megohm, and the net impedance of the A. C. voltage source will be considerably less than this value, even at full volume position. In orderto maintain the source impedance the resistor Re which is about 0.25 megohm is introduced. Without this resistor the ATC action is negligible at low volume control positions. As an average figure condenser C may be about 200 mi. for R1+R2=1 megohm. Assuming a voltage gain of 100 at maximum ATC action, this is equivalent to 0.02 mi. in series with 10,000 ohms shunted across the source ofaudio voltage. The maximum effective value of this condenser may be limited, if desired, by the proper choice of the value of C, as above noted. The minimum value of the resistance may; of course, be limited by placing another resistor (not shown) in series with C, outside the sphere of action of the ATC tube. Other circuit elements shown in the figure have their ordinary functions and values, and the latter need not be specified here.

Measurements on average 6D6 tubes give the following optimum screen voltages for several values of plate resistor and initial control grid bias. The screen voltage recorded-is that to the nearest volt below the actual voltage giving maximum gain.

TABLE Type 6D6 Values of E to give maximum gain for "alues of R and Ee noted. Plate supply= 250 volts E, =.75 volt R1 E112 Gain Megohm El1= 1.0 volt E 1.5 volts B -3.0 volts The described ATC system has a'certain limitation that is likewise possessed by the more elaborate QAVC, or muting, circuit thatit is intended to replace. This limitation is, that the circuit necessarily operates from the AVG output of the i amplifier and second detector rather than from the input of thereceiver. Accordingly, the apparent action of 'the circuit depends upon the sensitivity of the receiver, and in each particular case the improvement in periormance arising from adding the circuit to a receiver depends upon the relation between the sensitivityand the noise level at the place where the receiver is located. If the receiver is hypersensitive, noise along (without carrier) may possibly give enough AVC voltage to bias the ATC tube below cut-on.

Conversely with an insensitive receiver the ATC will operate on strong carriers well above the noise level, where it is not desired. A sensitivity control is thus a desirable adjunct to the ATC circuit, and vice versa; each adds to the utility of the other. A variation of the circuit that may be used is the reducing of the maximum ATC action to suit particular conditions by the use of a variable resistor at B1 (Fig. 3) in place of fixed one.

In the foregoing description the only tubes considered for ATC purposes are the R. F. pentodes such as the 58, and the 6B6, in which the signal and the control bias are placed upon the same control grid. An interesting modification is the use of a double grid tube in which the signal is placed upon one grid and the variable control bias upon the other. The 6A7, and similar tubes using five grids between cathode and plate, might be used in this manner, either with the signal on the grid adjacent the cathode and the bias on the fourth grid, or vice versa. The theoretical advantage to be gained in the multi-grid tube lies in the fact that gain maybe varied by means of a bias on one grid while the cut-off point on the second, or signal, grid is not appreciably changed. This permits "centering the signal upon the operating characteristic by the use of a properly chosen fixed bias upon the signal grid, and minimizes non-linear distortion over the whole range of ATC action. Such multi-grid tubes are well known, and their use will be clear from the circuit in Fig. 3.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made. without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combinationwith a source of audio frequency energy and an audio frequency amplifier,

a tone control device comprising an electron discharge tube provided with a cathode, an anode and at least one cold control electrode, an audio frequency attenuation path connected between the input electrodes of said audio amplifier, said path including a condenser in series with the input admittance of said control tube, a resistor of a predetermined magnitude in the anode circuit of said control tube, an impedance path connecting the control electrode and anode of the control tube, the impedance path including at least a condenser of a predetermined capacitive magnitude in a series with a resistor, and means for varying the gain of said control tube.

2. In combination with a source of audio frequency energy and an audio frequency amplifier, a tone control device comprising an electron discharge tube provided with a cathode, an anode and at least one cold control electrode, an audio frequency attenuation path connected between the input electrodes of said audio amplifier, said path" including" a condenser in series withcthe input admittance of said control tube, a resistor of a predetermined magnitude in the anode circuit of .said controltube, an impedance pathconnecting the control electrode and anode of the control tube, the impedance path including at least a condenser of a predetermined capacitive magniture in series with a resistor, and means for varying the bias of the control electrode of said control tube.

3. An electronic condenser of a variable type comprising an electron discharge tube provided with a cathode, an anode and at least one grid, a path connecting the grid and anode including a condenser of a predetermined magnitude in series with a resistor of a predetermined magnitude, the anode circuit load of said tube being purely resistive and of a magnitude having a predetermined relation to the magnitude of said condenser, and means for varying the gain of the tube.

4. An electronic condenser of a variable type comprising an electron discharge tube provided with a cathode, an anode and at least one grid, a path connecting the grid and anode including a condenser of a predetermined magnitude, the anode circuit load of said tube being purely resistive and of a magnitude having a predetermined relation to the magnitude of said condenser, and means for varying the bias of the grid with respect to the cathode, there being a resistor, of a predetermined magnitude with respect to the resistive anode load of said tube, in series with said condenser.

5. In combination with a source of audio frequency energy and an electron discharge tube repeater for said energy, an audio frequency attenuation path connected between the input electrodes of said repeater, said attenuation path including a pair of condensers and a resistor arranged in series, the resistor and one of said condensers being adjustable in magnitude to vary the frequency attenuation characteristic of said attenuation path, and said resistor and one condenser being provided by the input admittance of a tube whose grid and plate are connected by a condenser and resistor in series.

6. In a radio receiver including a source of signal energy, a demodulator and an audio frequency amplifier, an electron discharge tube having its input electrodes connected in shunt across the input electrodes of said audio amplifier, a path connecting the grid and anode of said electron discharge tube which includes a condenser in series with a resistor of a predetermined magnitude, and a resistor of a predetermined value in the anode circuit of said electron discharge tube, and means for varying the gain of said electron discharge tube as a function of variations in received signal carrier amplitude.

'7. In a radio receiver of the type including a signal amplifier network, a demodulator, an audio frequency amplifier, and an automatic gain control circuit for regulating the gain of said signal amplifier network in a sense such that the signal amplitude at the detector input is substantially uniform, an automatic tone control tube having its input electrodes connected in shunt across the input electrodes of said audio amplifier, the grid and plate of said tone control tube being connected by a path including a condenser in series with a resistor, and a resistive load in the plate circuit of said tone control tube, and means for varying the bias of the grid of said tone control tube in the same sense as the gain regulation of said signal amplifier network.

8. In a system as defined in claim '7, adjustable means for maintaining the cathode of said tone control tube at a positive potential with respect to the cathode of said audio amplifier,

thereby to vary the tone control action of said tone control tube.

9. In combination with a detector and audio frequency amplifier of a radio receiver, an automatic tone control tube, a path including a condenser in series with the input admittance of said tone control tube, said path being connected between the input electrodes of said audio amplifier, a condenser connected between the grid and'plate of said tone control tube for augmenting the capacitive effect of said input admittance, the plate load of said tone control tube being resistive, and a resistor in series with said augmenting condenser, said last named resistor being of a magnitude to augment the resistive effect of said tone control tube plate circuit in the said input admittance.

10. In combination with a detector and audio frequency amplifier of a radio receiver, an automatic tone control tube, a path including a condenser in series with the input admittance of said tone control tube, said path being connected between the input electrodes of said audio amplifier, a condenser connected between the grid and plate of said tone control tube for augmenting the capacitive effect of said input admittance, the plate load of said tone control tube being resistive, and a resistor in series with said augmenting condenser, said last named resistor being of a magnitude to augment the resistive effect of said tone control tube plate circuit in the said input admittance, and means for varying the potential difference between the input electrodes of said tone control tube as a function of the signal amplitude variation at the detector input.

11. In a wave transmission network including at least one wave transmission tube, a circuit for controlling the frequency response characteristic of the tube, said circuit comprising a second tube having its input admittance connected in series with a condenser between the first tube input electrodes, a resistive load in the plate circuit of the second tube, a condenser connected between the plate and wave input electrode of the second tube for augmenting the condensive effect of said admittance, a resistor in series with said condenser to augment the resistive effect of the said load in said admittance, and means for varying the second tube gain.

12. An electronic condenser comprising an electron discharge tube having at least a cathode, an anode and a cold electrode, a condenser connected between the cold electrode and anode for augmenting the condensive effect of the admittance between the cathode and said cold electrode, a resistive load connected to the anode, and a resistor, in series with the augmenting condenser, of a magnitude to augment the resistive effect of said load in said admittance.

13. An electronic condenser comprising an electron discharge tube having at least a cathode, an anode and a cold electrode, a condenser connected between the cold electrode and anode for augmenting the condensive effect of the admittance between the cathode and said cold electrode, a resistive load connected to the anode, a resistor, in series with the augmenting condenser, of a magnitude to augment the resistive effect of said load in said admittance, and means for varying an electrical characteristic of the tube for producing a desired change in the said condensive effect.

CHARLES TRAVIS. 

